Over the past two decades substantial effort has been directed to the creation and study of what are commonly called nanoparticles, despite the fact that numerous definitions of what qualify as so-called nanoparticles exist. Under the broadest definition, any particle having one dimension measuring less than 100 nanometers (nm) (or <100×10−9 m) can qualify as a “nanoparticle” despite the fact that other dimensions of that particle may be quite large. Even for a particle for which every dimension measures less than 100 nm, the designation of nanoparticle does not provide information related to particle shape or its fundamental properties, which may or may not differ from the solid material from which it is made. Additionally, prior to the widespread use of the term nanoparticle, very small particles, some of which would qualify under the standard definition of nanoparticles were often referred to as colloidal, which generally merely meant that they were small enough to exhibit Brownian motion, although whether that effect resulted from the particle size or other properties such as surface tension was rarely specified.
Today particles that qualify as nanoparticles under one or more traditional definition are used in multiple industrial, medical and consumer products, and interest in their properties and methods of production continues to increase.
Various processes to produce what are referred to as nanoparticles are known in the art. For example, U.S. Pat. No. 5,585,020, issued to Becker et al., teaches methods for creation of nanoparticles in what is considered a narrow size distribution (e.g., particles with an average diameter of 73 nm with a standard deviation of 23 nm). This method utilizes laser ablation of initial diameter target particles of less than 100 microns within an inert gas or vacuum system.
U.S. Pat. No. 7,374,730 teaches methods for creation of nanoparticles within organic liquid medium and recognizes the need for stabilizing agents such as surfactants or coating agents or other hydrocarbon materials capable of preventing coalescence of the nanoparticles or otherwise preventing the growth of the nanoparticles into larger entities.
U.S. Pat. No. 7,662,731 recognizes the need to prevent oxidation during laser sputtering/ablation, but solves this by carrying out the ablation in superfluid helium.
Picosecond ablation provides shorter pulses that reduce the time for ions to form and allows a method to control size, although the power output of picosecond ablation is generally significantly small, limiting quantities of material produced with relatively small ablation material plumes.
The shape of nanoparticles is also a significant characteristic and is a necessary characteristic in defining how a nanoparticle acts, interacts, or can be acted upon. Spherical particles are desirable for their uniform shape and repeatable characteristics.
Some nanoparticles can be grown into spheres through chemical reduction methods (e.g., silica), while production of spherical nanoparticles from other starting materials has traditionally been through a two step process. Typically, growth of nanoparticles from non-silica starting materials by the similar chemical reduction methods produce non-spherical shapes such as hedrons, platelets, rods, and other non-spherical shapes. While these methods provide good control for size, the resulting non-spherical shapes require further processing before they can become spherical in shape. Once the specifically shaped nanoparticles have been created, laser ablation is utilized to aggressively mill them into quasi-spherical and/or spherical shapes. This process produces what could be called scrap material that is knocked off of the original non-spherical particles, and in many instances this scrap lacks the intranuclear bonding energy to be cohesive in the carrier medium resulting in ion production. The spherical particles are then filtered to remove the ions and unwanted scrap. Although desired spherical nanoparticles are achieved by this method, the process is limited in its production capacity by the size of ablation field and by the batch process of precursor materials.
Due to the relatively recent advances in nanomaterial science and research, as well as, the identification of unique properties related to specific nanomaterials, standardizations for nanomaterial characteristics are continuing to develop. In the present application, the term nanoparticle will be used to refer to particles of any shape having its largest dimension less than 100 nm.
Significantly absent in the art are methods capable of producing high volume, uniformly sized, ionically stable, nanoparticles, and particularly spherical nanoparticles. Further absent from the art are methods for producing nanoparticles that can be suspended within a liquid solution and more particularly within a polar liquid solution without the need for surfactant or other stabilizing additives.